1. Field of the Invention
The present invention relates to projectors, and in particular to an automatic focusing system for use with front throw and rear screen slide projectors.
2. Description of the Prior Art
Many systems to provide auto focusing of a projected image of a slide projector have been proposed in the prior art. Generally, these systems require an initial, manual focusing of the first slide in a series, which then sets up a predetermined relationship between the film plane and the projection lens. Reflective mode sensors were utilized in an attempt to maintain said predetermined distance constant during "popping" of heated slides and slide changes. However, in order to operate effectively, the optical angle between the light source and sensor of these prior art systems were, of necessity, substantially greater than 90.degree., even 120.degree., in order to provide sufficient reflection. This very large inclusive optical angle would cause problems when glass slides, those in which the film is sandwiched between two thin plates of glass, and regular slides are intermixed in a series. The high optical angle of these prior systems caused reflection off of the front surface of one of the glass plates, and thus, did not maintain proper focus of all of the images during a specific series.